Oil or natural gas produced from oil and gas fields contains highly corrosive gases such as carbon dioxide (CO2) and hydrogen sulfide (H2S). The steel utilized in welded structures such as pipelines that convey these types of highly corrosive fluids is required to possess excellent resistance to corrosion. Many studies have been made of sulfide stress cracking (hereinafter referred to as “SSC”) caused by hydrogen sulfide and total surface corrosion caused by carbon dioxide gas in steel material for welded structures.
Adding Cr, for example, is known to lower the corrosion speed. Therefore in high-temperature carbon dioxide gas environments, martensitic stainless steel with an increased Cr content such as 13Cr steel is utilized in the steel pipeline material.
However, SSC occurs in martensitic stainless steel in environments containing trace amounts of hydrogen sulfide. Cracks caused by SSC quickly penetrate through a thick plate in a short time and are also a localized phenomenon, and thus enhancement of the ability to withstand SSC (hereinafter referred to as, “SSC resistance”) is even more important than improvement in overall resistance to corrosion.
Adding molybdenum and nickel in appropriate quantities to the martensitic stainless steel is effective in stabilizing the anti-corrosiveness of covering films in hydrogen sulfide environments to improve the SSC resistance. Patent document 1 discloses a technology for adding Ti, Zr, and rare earth metals (REM) to fix P, which weakens the SSC resistance, and thus lowers P in solid solution to essentially obtain a low P content.
Non-patent document 1 discloses a technology for lowering the C content in the base metal to inhibit a rise in hardness in sections affected by the welding heat (hereinafter, this “heat affected zone” will be referred to as “HAZ”) and thus improve the SSC resistance in the welded section.
In recent years, stress corrosion cracking (herein after referred to as “SCC”), is becoming a drastic problem in martensitic stainless steel used in high-temperature carbon dioxide gas environments (hereinafter referred to as “Sweet Environment”), which have high temperatures from approximately 80-200° C. and contain CO2 and chloride ions. SCC is a similar phenomenon to SSC in that cracks swiftly penetrate through thick plates in a short time and that they occur locally.
A technology for improving the stress corrosion cracking resistance (hereinafter referred to as “SCC resistance”) in the HAZ of martensitic stainless steel in Sweet environments is disclosed, for example, in patent document 2 as a method for producing a circular welded joint where the P content is limited within 0.010%.    [Patent document 1] JP1993-263137A    [Patent document 2] JP2006-110585A    [Non-patent document 1] M. Ueda et al.: Corrosion/96 Paper No. 58, Denver